The digital subscriber line (DSL) is a widespread and popular technology that provides high-speed broadband communications to businesses, homes, schools and other consumers over existing telephone lines (conventional twisted pair wire) without disturbing conventional voice telephony. One type of DSL is asymmetric DSL (ADSL), which is a frequency division multiplexing (FDM) scheme that places the upstream and downstream communications in separate, non-overlapping, frequency bands. The bit loading for a particular ADSL subchannel is determined during initialization of the communications session and depends on various transmission channel characteristics or properties of the line (conventional twisted pair wire) between the central office (CO) of the DSL service provider and the customer premises over the range of subchannel frequencies. These properties are also important for initially setting up DSL service at a customer premises.
These line characteristics or properties can be determined using a training sequence that relies upon bidirectional communications between the DSL transceiver at the CO and the DSL transceiver at the customer premises. Such bidirectional communications are often unavailable because a DSL transceiver is not located at the customer premises prior to installation. As a result, measurement of the transmission characteristics or line properties between a CO and a possible customer premises are performed using a single-ended line test (SELT) by measuring the characteristics from the CO end of the connection.
One SELT method to estimate the length and attenuation of a transmission line involves a technique generally called time domain reflectometry (TDR). With the TDR technique a pulse is transmitted on the line and the received echo signal is recorded. The received signal will contain one or several echoes that could come from: the far-end side of the line, bridged taps any cable gauge changes etc. A TDR methodology for loop qualification and characterization is described in Galli et al., “Loop Makeup Identification Via Single Ended Testing: Beyond Mere Loop Qualification”, IEEE J. Selected Areas in Communications, Vol. 20, No. 5 (June 2002), pp. 923-935. One difficulty with the traditional TDR method is that the reflected pulse can be heavily attenuated and be difficult to detect, as it is hidden by the rather broad outgoing pulse. To avoid this problem, the pulses can be filtered, but the Galli article suggests to instead subtract the outgoing pulse to get a distinct reflected pulse. Another problem with the traditional TDR method is that for short lines, the outgoing and reflected pulses are close to each other and are difficult to separate. For a very long line, on the other hand, the reflected pulse is heavily attenuated and can be hidden in the noise. As a result, only one pulse is observable in some traditional TDR measurements and it is impossible to know if it is a result of the line being very short or very long.
Current ADSL transceivers often include an integrated broadband SELT function (e.g., Ericsson IP DSLAM EDN110). This poses a challenge when trying to make use of the TDR technique since the ADSL transceiver was not designed for SELT in the first place. In fact, dedicated SELT instruments have metallic contact to the line unlike an ADSL transceiver, which has an Analog Front-End equipped with a transformer. More specifically, the echo transfer function contains a slowly decaying oscillating signal that depends on the transformer and the type of transmission line. In order to catch and amplify the weak far-end reflected pulse a carefully designed filter has to be used. However, a well-designed filter optimized for a cable length of around 1 km will not perform well for a cable length of 4 km, and vice versa. One solution to this problem is to use several filters optimized for different cable lengths, but this solution is expensive and inaccurate.